Group 13 nitride composite substrate semiconductor device, and method for manufacturing group 13 nitride composite substrate

ABSTRACT

Provided are a group 13 nitride composite substrate allowing for the production of a semiconductor device suitable for high-frequency applications while including a conductive GaN substrate, and a semiconductor device produced using this substrate. The group 13 nitride composite substrate includes a base material of an n-conductivity type formed of GaN, a base layer located on the base material, being a group 13 nitride layer having a resistivity of 1×10 6  Ω·cm or more, a channel layer located on the base layer, being a GaN layer having a total impurity density of 1×10 17 /cm 3  or less, and a barrier layer that is located on the channel layer and is formed of a group 13 nitride having a composition Al x In y Ga 1-x-y N (0≤x≤1, 0≤y≤1).

This application is a Continuation of, and claims priority under 35U.S.C. § 120 to, U.S. patent application Ser. No. 14/657,704, filed Mar.13, 2015, which was a Continuation under U.S.C. § 120 of InternationalPatent Application No. PCT/JP2014/064420, filed May 30, 2014, whichclaimed priority under 35 U.S.C. § 119(e) to U.S. Provisional PatentApplication No. 61/831,671, filed Jun. 6, 2013, the entireties of whichare incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a semiconductor device, andparticularly, to a group 13 nitride composite substrate allowing for theproduction of semiconductor devices suitable for high-frequencyapplications.

BACKGROUND ART

Nitride semiconductors, which have high breakdown electric field andhigh saturation electron velocity, have been attracting attention as thenext-generation semiconductor materials for high-frequency/high-powerdevices. In particular, a multi-layer structure, which is formed bylaminating a layer composed of AlGaN and a layer composed of GaN,produces a high two-dimensional electron gas (2DEG) at a laminationinterface (hetero interface) owing to large polarization effects(spontaneous polarization effect and piezo polarization effect) inherentin nitride materials, and thus, high electron mobility transistors(HEMTs) including such a multi-layer structure as a substrate have beenvigorously developed (for example, see Non-Patent Document 1).

HEMTs, which are operated under the conditions of high power and highfrequency (100 W or more, 2 GHz or more) such as ones for mobile phonebase stations, are desirably produced using materials having heatresistant as low as possible to limit a temperature rise of a device dueto heating. Contrastingly, HEMTs, which perform a high-frequencyoperation, are desirably produced using highly insulating materialsbecause they need to reduce parasitic capacitance as much as possible.In the production of a device that satisfies the above-mentionedrequirements using a nitride semiconductor, a semi-insulating SiCsubstrate having a resistivity as high as 1×10⁸ Ω·cm or more is used asa base substrate because such a substrate allows for the deposition of agood nitride film.

It is proposed to deposit an insulating AlN film on a conductive SiCsubstrate by the method such as the hydride vapor phase epitaxy method(HVPE method) or the MOCVD method and use it as a base substrate (forexample, see Non-Patent Document 2).

In the technique disclosed in Non-Patent Document 2, however, since thecrystal quality of a nitride epitaxial film formed on the base substratedepends on the quality of the AlN film formed by the HVPE method, thequality of the AlN film is required to be improved for improved qualityof the nitride epitaxial film. Unfortunately, it is difficult to controlthe deposition of the AlN film by the HYPE method in such a way that thecrystal quality (such as dislocation density) becomes uniform over theentire wafer in the deposition, leading to inplane variations incharacteristics of an epitaxial film, further, of a device.

The approach capable of achieving effects similar to those in the caseof using a semi-insulating SiC substrate with the use of a basesubstrate including a vanadium-doped semi-insulating SiC film formed ona conductive SiC substrate has been known (for example, see PatentDocument 1).

In recent years, gallium nitride (GaN) substrates expected to haveimproved performance and reliability have been in practical use as thebase substrate for HEMT device. The approach of manufacturing a GaNsubstrate by the gas phase process or liquid phase process has beenknown (for example, see Patent Documents 2 and 3).

As described above, in use of a nitride semiconductor for high-frequencyapplication, it is desirable that the substrate be free from parasiticcapacitance. Thus, a semi-insulating GaN substrate is desirably usedeven in the use of a GaN substrate, but now, a semi-insulating GaNsubstrate is expensive and is hard to obtain. In contrast, a conductivegallium nitride substrate is relatively inexpensive and is easy toobtain because conductive gallium nitride substrates are in massproduction for vertical LDs.

An approach of forming a carbon (C) doped GaN layer on a conductive GaNsubstrate to obtain a GaN substrate that can be used for high-frequencyapplications, in which the above-mentioned problem is taken intoconsideration, has been known (for example, see Patent Document 4). Inthe technique disclosed in Patent Document 4, however, the Cconcentration of an electron transit layer becomes higher, which makesit difficult to improve device performance.

There is a known technique of doping zinc (Zn) to obtain ahigh-resistance nitride single crystal (for example, see Patent Document5).

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: Japanese Patent Application Laid-Open No.    2010-062168-   Patent Document 2: Japanese Patent No. 3631724-   Patent Document 3: International Publication No. 2010/084675-   Patent Document 4: Japanese Patent Application Laid-Open No.    2012-199398-   Patent Document 5: Japanese Patent No. 5039813

Non-Patent Documents

-   Non-Patent Document 1: “Highly Reliable 250 W GaN High Electron    Mobility Transistor Power Amplifier”, T. Kikkawa, Japanese Journal    of Applied Physics, Vol. 44, No. 7A, 2005, pp. 4896-4901.-   Non-Patent Document 2: “A 100-W High-Gain AlGaN/GaN HEMT Power    Amplifier on a Conductive N—SiC Substrate for Wireless Bass Station    Applications”, M. Kanamura, T. Kikkawa, and K. Joshin, Tech. Dig. of    2004 IEEE International Electron Device Meeting (IEDM2008), pp.    799-802.

SUMMARY OF INVENTION

The present invention has been made in view of the above-mentionedproblem, and has an object to provide a group 13 nitride compositesubstrate allowing for the production of a semiconductor device suitablefor high-frequency application while including a conductive GaNsubstrate, and a semiconductor device produced using the group 13nitride composite substrate.

To solve the above-mentioned problem, in a first aspect of the presentinvention, a group 13 nitride composite substrate includes: a basematerial of an n-conductivity type formed of GaN; a base layer locatedon the base material, the base layer being a group 13 nitride layerhaving a resistivity of 1×10⁶ Ω·cm or more; a channel layer located onthe base layer, the channel layer being a GaN layer having a totalimpurity concentration of 1×10¹⁷/cm³ or less; and a barrier layerlocated on the channel layer, the barrier layer being formed of a group13 nitride having a composition Al_(x)In_(y)Ga_(1-x-y)N (0≤x≤1, 0≤y≤1).

In a second aspect of the present invention, in the group 13 nitridecomposite substrate according to the first aspect, the base layer is aZn-doped GaN layer being a GaN layer containing Zn doped at a density of1×10¹⁸/m³ or more and 2×10¹⁹/cm³ or less.

In a third aspect of the present invention, in the group 13 nitridecomposite substrate according to the first aspect, the base layer is aC-containing GaN layer being a GaN layer containing C at a density of8×10¹⁶/cm³ or more and 3×10¹⁸/cm³ or less.

In a fourth aspect of the present invention, in the group 13 nitridecomposite substrate according to the first aspect, the base layer is anAlGaN layer formed of Al_(p)Ga_(1-p)N (0.1≤p≤0.98).

In a fifth aspect of the present invention, a semiconductor deviceincludes the group 13 nitride composite substrate according to the firstaspect, a source electrode and a drain electrode that are located on thebarrier layer of the group 13 nitride composite substrate and have ohmiccontact with the barrier layer, and a gate electrode located on thebarrier layer of the group 13 nitride composite substrate and hasSchottky contact with the barrier layer.

In a sixth aspect of the present invention, a method for manufacturing agroup 13 nitride composite substrate includes: a base layer forming stepof forming, on a base material of an n-conductivity type formed of GaN,a base layer being a group 13 nitride layer having a resistivity of1×10⁶ Ω·cm or more; a channel layer forming step of forming, on the baselayer, a channel layer being a GaN layer having a total impurityconcentration of 1×10¹⁷/cm³ or less; and a barrier layer forming step offorming, on the channel layer, a barrier layer formed of a group 13nitride having a composition Al_(x)In_(y)Ga_(1-x-y)N (0≤x≤1, 0≤y≤1).

In a seventh aspect of the present invention, in the method formanufacturing a group 13 nitride composite substrate according to thesixth aspect, the base layer forming step is a Zn-doped GaN layerforming step of forming, as the base layer, a GaN layer containing Zndoped at a density of 1×10¹⁸/cm³ or more and 2×10¹⁹/cm³ or less.

In an eighth aspect of the present invention, in the method formanufacturing a group 13 nitride composite substrate according to thesixth aspect, the base layer forming step is a C-containing GaN layerforming step of forming, as the base layer, a GaN layer containing C ata density of 8×10¹⁶/cm³ or more and 3×10¹⁸/cm³ or less.

In a ninth aspect of the present invention, in the method formanufacturing a group 13 nitride composite substrate according to thesixth aspect, the base layer forming step is an AlGaN layer forming stepof forming, as the base layer, an AlGaN layer of Al_(p)Ga_(1-p)N(0.1≤p≤0.98).

According to the first to ninth aspects, a group 13 nitride compositesubstrate allowing for the production of a semiconductor device suitablefor high-frequency applications while including a conductive GaNsubstrate as a base material, and further, the semiconductor device canbe obtained. The semiconductor device has high mobility while thecapacitance between gate and source electrodes (gate-source capacitance)thereof is reduced.

In particular, according to the second to fourth and seventh to ninthaspects, a group 13 nitride composite substrate allowing for theproduction of a semiconductor device suitable for high-frequencyapplications while including a conductive GaN substrate as a basematerial, and further, the semiconductor device can be obtained. Thesemiconductor device has a mobility as high as 1000 cm²/V·s or morewhile the gate-source capacitance thereof is reduced to less than 0.1pF.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 schematically illustrates the cross-sectional structure of a HEMTdevice 20 including a group 13 nitride composite substrate 10.

EMBODIMENT FOR CARRYING OUT THE INVENTION

The group numbers of the periodic table in this specification areaccording to the explanation of group numbers 1 to 18 in thenomenclature of inorganic chemistry revised in 1989 by the internationalunion of pure applied chemistry (IUPAC). Group 13 refers to, forexample, aluminum (Al), gallium (Ga), and indium (In), and group 15refers to, for example, nitrogen (N), phosphorous (P), arsenic (As), andantimony (Sb).

<Configurations of Composite Substrate and HEMT Device>

FIG. 1 schematically illustrates the cross-sectional structure of a HEMTdevice 20 as an embodiment of a semiconductor device according to thepresent invention, which includes a group 13 nitride composite substrate10 as an embodiment of a group 13 nitride (group III nitride) compositesubstrate according to the present invention.

The group 13 nitride composite substrate 10 includes a base materialseed substrate) 1, a base layer (high-resistance layer) 2 (2A, 2B, or2C), a channel layer (low-impurity layer) 3, and a barrier layer 4. TheHEMT device 20 includes a source electrode 5, a drain electrode 6, and agate electrode 7 disposed on the group 13 nitride composite substrate 10(on the barrier layer 4). The ratios of the respective layers in FIG. 1do not reflect the actual ones. The configuration of the group 13nitride composite substrate 10, in which the barrier layer 4 is disposedon the channel layer 3, may be referred to as a HEMT structure below.

The base material 1 is a GaN substrate that has a resistivity of 1 Ω·cmor less, has an n-conductivity type, and has a (0001) plane orientation.Although the thickness of the base material 1 is not particularlylimited, it is preferably about several hundreds of μm to several mm inconsideration of, for example, ease of handling. Bulk GaN produced by,for example, a known technique such as the HVPE method may be used asthe base material 1.

The base layer 2 is a high-resistance (semi-insulating) group 13 nitridelayer having a resistivity of 1×10⁶ Ω·cm or more. The base layer 2 ispreferably provided with a thickness of 8 μm or more, and morepreferably, with a thickness of 10 μm or more and 200 μm or less.

The base layer 2 is preferably any one of a Zn-doped GaN layer 2A, aC-containing GaN layer 2B, and an AlGaN layer 2C. Each of the layerswill be described in detail below.

The channel layer 3 is a GaN layer having a total impurity concentrationof 1×10¹⁷/cm³ or less, which is formed by the MOCVD method. The channellayer 3 has an impurity concentration smaller than, at least, that ofthe base layer 2.

Typical impurity in the channel layer 3 are C. Thus, the channel layer3, which has a C concentration of less than 1×10¹⁷/cm³, can be virtuallyregarded to have a total impurity concentration of 1×10¹⁷/cm³ or less.The channel layer 3 is preferably provided with a thickness of 0.05 μmor more and 5 μm or less.

The barrier layer 4 is a group 13 nitride layer having a compositionAl_(x)In_(y)Ga_(1-x-y)N (0≤x≤1, 0≤y≤1), which is formed by the MOCVDmethod. The barrier layer 4 is preferably formed to have a thickness of5 to 30 nm.

The source electrode 5 and the drain electrode 6 are metal electrodeseach having a thickness of about ten and several nm to a hundred andseveral tens of nm. The source electrode 5 and the drain electrode 6have ohmic contact with the barrier layer 4.

The source electrode 5 and the drain electrode 6 are preferably formedas multilayer electrodes of, for example, Ti/Al/Ni/Au. In such a case,the Ti film, Al film, Ni film, and Au film preferably have thicknessesof about 10 to 50 nm, 50 to 200 nm, 10 to 50 nm, and 500 to 1000 nm,respectively.

The gate electrode 7 is a metal electrode having a thickness of aboutten and several nm to a hundred and several tens of nm. The gateelectrode 7 has Schottky contact with the barrier layer 4.

The gate electrode 7 is preferably formed as a multilayer electrode of,for example, Pd/Au. In such a case, the Pd film and the Au filmpreferably have thicknesses of about 5 to 50 nm and 50 to 500 nm,respectively.

Providing the above-mentioned configuration, specifically, providing thebase layer 2 being a high-resistance layer on the base material 1 beinga conductive GaN substrate and then sequentially providing the channellayer 3 being a low-impurity layer and the barrier layer 4 thereon,allows the HEMT device 20 according to this embodiment to have amobility as high as 1000 cm²/V·s or more while the gate-sourcecapacitance is reduced to less than 0.1 pF. These characteristic valuesare preferable ones so that the HEMT device 20 can be used forhigh-frequency applications. In particular, since the gate-sourcecapacitance becomes parasitic capacitance and degrades high-frequencycharacteristics, the capacitance preferably has a small value when theHEMT device 20 is used for high-frequency applications.

The HEMT device 20 according to this embodiment can be preferably usedfor high-frequency applications while it includes a conductive GaNsubstrate as the base material 1. The group 13 nitride compositesubstrate 10 according to this embodiment allows for the production of aHEMT device to be preferably used for high-frequency applications whileit includes a conductive GaN substrate as the base material 1.

<Detailed Configuration of Base Layer>

As described above, the base layer 2 is preferably any one of theZn-doped GaN layer 2A, C-containing GaN layer 2B, and AlGaN layer 2C.Each of these layers will now be described in detail.

The Zn-doped GaN layer 2A is a GaN layer formed by doping Zn (zinc) bythe flux method (sodium flux method). The Zn-doped GaN layer 2Apreferably has a Zn concentration of 1×10¹⁸/cm³ or more and 2×10¹⁹/cm³or less. In such a case, resistivity is 1×10⁷ Ω·cm or more, mobility is1150 cm²/V·s or more, and the gate-source capacitance falls below 0.1pF.

The Zn-doped GaN layer 2A, whose Zn concentration is set to be less than1×10¹⁸/cm³, has higher mobility but undesirably has a gate-sourcecapacitance exceeding 0.1 pF. The Zn concentration set to be more than2×10¹⁹/cm³ undesirably leads to a small mobility. It is conceivable thatmobility will decrease when the Zn concentration exceeds 2×10¹⁹/cm³because the crystallinity of the Zn-doped GaN layer 2A decreases and thecrystallinity of the channel layer 3 accordingly decreases as well.

The C-containing GaN layer 2B is a GaN layer formed to intentionallycontain C (carbon) as impurities by the metal organic chemical vaporphase deposition method (MOCVD method). The C-containing GaN layer 2Bpreferably has a C concentration of 8×10¹⁶/cm³ or more and 3×10¹⁸/cm³ orless. In such a case, resistivity is 3×10⁶ Ω·cm or more, mobility is1250 cm²/V·s or more, and the gate-source capacitance falls below 0.1pF.

The C-containing GaN layer 2B, whose C concentration is set to be lessthan 8×10¹⁶/cm³, has higher mobility but undesirably has a gate-sourcecapacitance exceeding 0.1 pF. The C concentration set to be more than3×10¹⁸/cm³ undesirably leads to a small mobility. It is conceivable thatmobility will decrease when the C concentration exceeds 3×10¹⁸/cm³because the crystallinity of the C-containing GaN layer 2B decreases andthe crystallinity of the channel layer 3 accordingly decreases as well.

The AlGaN layer 2C is a layer formed of Al_(p)Ga_(1-p)N by the MOCVDmethod. It is preferable that 0.1≤p≤0.98. In such a case, resistivity is2×10⁶ Ω·cm or more, mobility is 1050 cm²/V·s or more, and thegate-source capacitance falls below 0.1 pF.

The AlGaN layer 2C, whose Al compositional ratio p is set to be lessthan 0.1, has higher mobility but undesirably has a gate-sourcecapacitance exceeding 0.1 pF. The Al compositional ratio set to be morethan 0.98 undesirably leads to a small mobility. It is conceivable thatmobility will decrease when the Al compositional ratio exceeds 0.98because minute cracks appear in the channel layer and the crystallinityof the channel layer accordingly decreases.

As described above, the HEMT device 20 according to this embodiment canachieve a mobility as high as 1000 cm²V·s or more even if the base layer2 is formed as any one of the Zn-doped GaN layer 2A, the C-containingGaN layer 2B, and the AlGaN layer 2C, whereas the gate-sourcecapacitance is reduced down to less than 0.1 pF.

<Procedure of Producing Composite Substrate and HEMT Device>

Next, the procedure of producing the group 13 nitride compositesubstrate 10 and the HEMT device 20 that have the configurationsdescribed above will be described. Since it is preferable in thisembodiment that any one of the Zn-doped GaN layer 2A, the C-containingGaN layer 2B, and the AlGaN layer 2C be formed on the base material 1 asthe base layer 2 as described above, the methods of forming the Zn-dopedGaN layer 2A, the C-containing GaN layer 2B, and the AlGaN layer 2C willbe individually described as the method of forming the base layer 2, andthen, the formation of the channel layer 3 and the barrier layer 4 onthe base layer 2 will be described.

(Formation of Zn-Doped GaN Layer)

The Zn-doped GaN layer 2A is produced by the flux method. Specifically,a GaN substrate being the base material 1 is prepared. Then, the basematerial 1 being a seed crystal, 20 to 70 g of metal Ga, 40 to 120 g ofmetal Na, and 0.1 to 5 g of metal Zn are charged into an aluminacrucible. Subsequently, the alumina crucible is placed in aheat-resistant metal growing vessel and is then sealed.

Then, in the heat-resistance, pressure-tight crystal growing furnacewhose furnace temperature has been set to 800 to 900° C. and whosefurnace pressure has been set to 3 to 10 MPa, into which a nitrogen gashas been introduced, the growing vessel is held for 20 to 100 hourswhile being horizontally rotated. With this holding, a GaNsingle-crystal layer containing doped Zn is deposited to have athickness of about 100 to 500 μm on the base material 1 while a meltcontaining the metal Ga, the metal Na, and the metal Zn is beingstirred.

The furnace is cooled slowly to room temperature, and then, the basematerial 1 on which a Zn-doped GaN single crystal layer has deposited(composite substrate) is taken out of the alumina crucible.

Subsequently, the surface of the formed Zn-doped GaN single crystallayer is planarized with diamond abrasive grains such that the layer hasa thickness of 10 to 100 μm. This completes the formation of theZn-doped GaN layer 2A.

If the single crystal layer grown by the flux method has a thickness ofless than 10 μm, it is difficult to planarize the surface of the singlecrystal layer and regulate the thickness thereof at a constant value.Therefore, the single crystal layer grown by the flux method preferablyhas a thickness of 10 μm or more.

(Formation of C-Containing GaN Layer) The C-containing GaN layer 2B isformed by the MOCVD method. In the formation of the C-containing GaNlayer 2B, a known MOCVD furnace is used that is configured such that itsreactor is supplied with at least a metal organic (MO) source gas for Ga(TMG), an ammonia gas being a source gas of N, a hydrogen gas, and anitrogen gas. Needless to say, the MOCVD furnace may be configured to besupplied with other source gas.

Specifically, a GaN substrate being the base material 1 is firstprepared and is placed on the susceptor provided in the reactor. Then,the susceptor is heated to set the base material 1 to a predeterminedtemperature (C-containing GaN layer forming temperature) of 1000° C. orhigher and 1150° C. or lower. At the same time, while keeping thereactor pressure at a predetermined value of 10 kPa or more and 50 kPaor less, the supply of TMG and an ammonia gas being source gases, andfurther, a carrier gas is adjusted such that a gas ratio of group 15 togroup 13 has a predetermined value of 100 or more and 2000 or less. As aresult, the C-containing GaN layer 2B having a desired C concentrationis formed on the surface of the base material 1.

In the formation of a GaN layer by the MOCVD method, the C concentrationthereof varies depending on a value of the gas ratio of group 15 togroup 13. This phenomenon is employed in the formation of theC-containing GaN layer 2B in this embodiment. That is, the GaN layer iscaused to contain a desired concentration of C by appropriatelyadjusting the reactor pressure and the gas ratio of group 15 to group 13in the formation of a GaN layer.

For example, the C concentration of the C-containing GaN layer 2B is5×10¹⁶/cm³ in the case where the reactor pressure is set to 100 kPa andthe gas ratio of group 15 to group 13 is set to 1000, and the Cconcentration of the C-containing GaN layer 2B is 5×10¹⁸/cm³ in the casewhere the reactor pressure is set to 10 kPa and the gas ratio of group15 to group 13 is set to 100. The C concentration of the C-containingGaN layer 2B can vary depending on the value of the gas ratio of group15 to group 13 because the C supply amount varies depending on thesupply amount of a group 13 gas and the stability of the C element of aGaN crystal is subject to temperature and pressure.

In this embodiment, the gas ratio of group 15 to group 13 is a ratio(molar ratio) of the supply amount of a group 15 source to the supplyamount of a group 13 source. In the formation of the C-containing GaNlayer 2B, the molar ratio of the supply amount of an ammonia gas beingan N source to the supply amount of TMG being a Ga source corresponds tothe gas ratio of group 15 to group 13.

(Formation of AlGaN Layer)

The AlGaN layer 2C is formed by the MOCVD method, similarly to theformation of the C-containing GaN layer 2B. Thus, in the formation ofthe AlGaN layer 2C, an MOCVD furnace similar to that in the formation ofthe C-containing GaN layer 2B can be used if it can be also suppliedwith a metal organic (MO) source gas for Al (TMA). Needless to say, theMOCVD furnace may be configured to be supplied with other source gas.

Specifically, a GaN substrate being the base material 1 is firstprepared and is placed on the susceptor provided in the reactor. Then,the susceptor is heated to set the base material 1 to a predeterminedtemperature (AlGaN layer forming temperature) of 1050° C. or higher and1200° C. or lower. At the same time, while keeping the reactor pressureat a predetermined value of 5 kPa or more and 30 kPa or less, the supplyof TMG and TMA being source gases and an ammonia gas, and further, acarrier gas is adjusted such that a gas ratio of group 15 to group 13has a predetermined value of 500 or more and 5000 or less. As a result,the AlGaN layer 2C is formed on the surface of the base material 1.

To set the Al compositional ratio p of the AlGaN layer 2C to apredetermined value, the ratio of the Al source gas (TMA) to the group13 source gas, namely, the ratio of the flow of TMA to the flow of theentire group 13 source gas (a total sum of the flow of TMA and TMG), maybe adjusted to agree with a desired compositional ratio. If such a flowrate ratio is represented as TMA/(TMA+TMG) ratio, for example, the Alcompositional ratio p of the AlGaN layer 2C is 0.1 for the TMA/(TMA+TMG)ratio set to 0.1, and is 0.98 for the TMA/(TMA+TMG) ratio set to 0.98.

(Formation of Channel Layer and Subsequent Ones)

After the formation of the base layer 2 by any of the approachesdescribed above, the channel layer 3 and the barrier layer 4 are formedin sequence. The channel layer 3 and the barrier layer 4 are formed bythe MOCVD method. Preferably, the channel layer 3 and the barrier layer4 are successively formed in a single MOCVD furnace. To form theC-containing GaN layer 2B or AlGaN layer 2C as the base layer 2, it ispreferable to sequentially form layers including these layers in asingle MOCVD furnace.

To form the channel layer 3 and the barrier layer 4, a known MOCVDfurnace is used that is configured such that its reactor can be suppliedwith metal organic (MO) source gases for group 13 elements (Ga, Al, In)(TMG, TMA, TMI), an ammonia gas being the source gas of nitrogen (N), ahydrogen gas, and a nitrogen gas.

To form the channel layer 3, the composite substrate after the formationof the base layer 2 is first placed on the susceptor provided in thereactor. Then, the susceptor is heated to set the composite substrate toa predetermined temperature (channel layer forming temperature) of 1000°C. or higher and 1150° C. or lower. At the same time, while keeping thereactor pressure at a predetermined value of 50 kPa or more and 100 kPaor less, the supply of TMG and an ammonia gas being source gases, and acarrier gas is adjusted such that the gas ratio of group 15 to group 13has a predetermined value of 1000 or more and 5000 or less. As a result,the channel layer 3 is formed.

To form the barrier layer 4 subsequent to the formation of the channellayer 3, the composite substrate after the formation of the channellayer 3 is set to a predetermined temperature (barrier layer formingtemperature) of 1050° C. or higher and 1200° C. or lower. At the sametime, while keeping the reactor pressure at a predetermined value of 5kPa or more and 30 kPa or less, the supply of TMG, TMA, TMI, and anammonia gas being source gases and a carrier gas is adjusted dependingon the composition of the barrier layer 4 such that the gas ratio ofgroup 15 to group 13 is 5000 or more and 50000 or less. As a result, thebarrier layer 4 is formed.

As the result of the formation of the barrier layer 4, the group 13nitride composite substrate 10 according to this embodiment has beenobtained.

After the formation of the group 13 nitride composite substrate 10, theHEMT device 20 is formed using this substrate. The following steps willbe performed by a known technique.

First, a multilayer film of Ti/Al/Ni/Au is formed at locations forformation on the barrier layer 4 by the photolithography process and thevacuum deposition method, thereby forming a multilayer metal that willserve as the source electrode 5 and the drain electrode 6.

Subsequently, to provide good ohmic property to the source electrode 5and the drain electrode 6, the group 13 nitride composite substrate 10on which the source electrode 5 and the drain electrode 6 have beenformed is heat-treated for several tens of seconds in a nitrogen gasatmosphere at a predetermined temperature of 650 to 1000° C.

Subsequently, a multilayer film of Pd/Au is formed at a location forformation on the barrier layer 4 by the photolithography process and thevapor deposition method, thereby forming a multilayer metal that willserve as the gate electrode 7.

As described above, in this embodiment, the base layer being ahigh-resistance layer is provided on the base material being aconductive GaN substrate, and then, the channel layer being alow-impurity layer and the barrier layer are provided thereon insequence, thereby forming a HEMT device having a mobility as high as1000 cm²/V·s or more while the gate-source capacitance thereof isreduced to less than 0.1 pF. In other words, this embodiment enables agroup 13 nitride composite substrate allowing for the production of aHEMT device suitable for high-frequency applications while including aconductive GaN substrate as a base material, and further enables a HEMTdevice suitable for high-frequency applications.

EXAMPLES Example 1

In this example, a Zn-doped GaN layer 2A was formed as a base layer 2 toproduce a group 13 nitride composite substrate 10, and further, thegroup 13 nitride composite substrate 10 was used to produce a HEMTdevice 20. Then, several characteristics were evaluated during and afterthe production.

More specifically, in this example, six types of group 13 nitridecomposite substrates 10 were produced on different conditions for the Znconcentration of the Zn-doped GaN layer 2A, and HEMT devices 20 for therespective substrates 10 were produced (Nos. 1-1 to 1-6). In thefollowing description, a GaN layer from which Zn is not detected may bereferred to as a Zn-doped GaN layer 2A for convenience sake.

A so-called multi-patterning process capable of producing a large numberof devices from a single mother substrate was employed in the productionof the HEMT devices 20.

In the production of a sample under any condition, first, a 4-in.-diam.conductive GaN substrate of an n-conductivity type having a (0001) planeorientation was prepared as the mother substrate that serves as the basematerial 1. The resistivity of this GaN substrate was 0.1 Ω·cm.

Then, the conductive GaN substrate, a metal Ga, a metal Na, and a metalZn were charged into an alumina crucible. On this occasion, the chargeamounts of the metal Ga and metal Na were 45 g and 66 g, respectively,whereas the charge amount of the metal Zn was varied for each conditionto vary Zn concentration. Specifically, the charge amounts of thesamples 1-1 to 1-6 were 0 g (no charge), 0.1 g, 0.2 g, 0.5 g, 2 g, and 5g, respectively.

After that, the alumina crucibles on the respective conditions were putinto the heat-resistant metal growing vessel and sealed. Further, thegrowing vessel was held while being horizontally rotated for about 10hours in the crystal growing furnace, into which a nitrogen gas was tobe introduced, under the condition that the furnace temperature was 900°C. and the furnace pressure was 5 MPa.

The conductive GaN substrate was taken out of the alumina crucible afterthe growth, and then, it was confirmed that a GaN single crystal wasdeposited with a thickness of about 150 μm on the (0001) plane thereofan each condition.

Then, the surface of the GaN single crystals formed on the conductiveGaN substrate was ground with diamond abrasive grains and was planarizedto have a thickness of 25 μm. As a result, six types of compositesubstrates were obtained, each of which had the Zn-doped GaN layer 2A asthe base layer 2 on the conductive GaN substrate. In any of the samples,no crack was found on the surface of the Zn-doped GaN layer 2A.

For each of the composite substrates, the Zn concentration of theZn-doped GaN layer 2A was identified by SIMS, and the resistivity of theZn-doped GaN layer 2A was measured by the van der Pauw method.

Table 1 shows the Zn concentration and resistivity of the Zn-doped GaNlayer 2A (base layer) for each sample.

TABLE 1 Zn concentration Resistivity Mobility Capacitance Sample No.[/cm³] [Ω · cm] [cm²/V · s] [pF] 1-1 <1 × 10¹⁶  1 × 10¹ 1850 120 1-2 5 ×10¹⁷ 3 × 10⁶ 1750 0.5 1-3 1 × 10¹⁸ 1 × 10⁷ 1600 <0.1 1-4 1 × 10¹⁹ 3 ×10⁷ 1200 <0.1 1-5 2 × 10¹⁹ 5 × 10⁷ 1150 <0.1 1-6 3 × 10¹⁹ 1 × 10⁸ 650<0.1

As shown in Table 1, the Zn concentration of the Zn-doped GaN layer 2Ahad a larger value with a larger charging amount of the metal Zn. It wasconfirmed that the sample 1-1 actually contained no Zn because themeasured value of Zn concentration was less than the lower detectionlimit, 1×10¹⁶/cm³.

The resistivity of the Zn-doped GaN layer 2A was 1×10⁶ Ω·cm or moreexcept for the sample 1-1 and had a larger value with a higher Znconcentration. In particular, the values of 1×10⁷ Ω·cm or more wereobtained in the samples 1-3 to 1-6. This confirmed that the Zn-doped GaNlayer 2A was formed as a semi-insulating layer in the samples 1-2 to1-6.

Each of the thus obtained composite substrates was placed in thesusceptor in the reactor of the MOCVD furnace. After the internal gas ofthe reactor was replaced with a nitrogen gas, the reactor pressure wasset to 100 kPa, thereby forming an atmosphere in hydrogen/nitrogen mixedflow state. Subsequently, the susceptor was heated to raise thetemperature of the substrate. When the susceptor temperature reached1100° C., a TMG gas and an ammonia gas were introduced into the reactor,thereby forming a GaN layer as the channel layer 3 to have a thicknessof 2 μm.

In the formation of the channel layer 3, a hydrogen gas was used as thebubbling gas for a metal organic source and the carrier gas. The gasratio of group 15 to group 13 was set to 2000.

The C concentrations of the samples, which have undergone the process upto the formation of the channel layer 3 on the same conditions, weremeasured by SIMS, so that the C concentration of each sample was about2×10¹⁶/cm³. This confirmed that the channel layer 3 was formed as alow-impurity layer. The lower detection limit of the C concentration inthe SIMS measurement was 1×10¹⁶/cm³.

After the channel layer 3 was obtained, then, the susceptor temperaturewas continuously kept at 1100° C., and the reactor pressure was set to10 kPa. Subsequently, TMG, TMA, and an ammonia gas were introduced intothe reactor at a predetermined flow rate ratio, thereby forming anAl_(0.2)Ga_(0.8)N layer as the barrier layer 4 to have a thickness of 25nm. In the formation of the barrier layer 4, a hydrogen gas was used asthe bubbling gas for the metal organic source and the carrier gas. Thegas ratio of group 15 to group 13 was 5000.

After the formation of the barrier layer 4, the susceptor temperaturewas lowered to around room temperature, and the internal gas of thereactor was returned to atmospheric pressure. Then, the reactor wasopened to the air, thereby taking out the produced group 13 nitridecomposite substrate 10.

Then, the mobility of the HEMT structure formed on the group 13 nitridecomposite substrate 10 was measured by Hall measurement (van der Pauwmethod).

Specifically, a 6 mm×6 mm square specimen was cut out from each of thesix types of group 13 nitride composite substrates 10. Subsequently, 0.5mm×0.5 mm square Ti/Al electrodes were deposited on the four corners ofthe specimen, and were then annealed for one minute at 600° C. in anitrogen gas. Then, the temperature was lowered to room temperature,thereby obtaining samples for measurement. After confirming that ohmiccontact has been provided between the Ti/Al electrodes and the HEMTstructure, the mobility of the HEMT structure was measured by Hallmeasurement. Table 1 also shows the measurement results of the mobility.

Subsequently, the HEMT device 20 was manufactured using the group 13nitride composite substrate 10. The HEMT device 20 was designed to havea gate width of 1 mm, a source-gate distance of 1 μm, a gate-draindistance of 7.5 μm, and a gate length of 1.5 μm.

In the production of the HEMT device 20, prior to the formation of theelectrodes, a SiN film (not shown) was formed with a thickness of 100 nmas a passivation film on the group 13 nitride composite substrate 10 (onthe barrier layer 4).

Subsequently, the SiN film formed at the locations at which the sourceelectrode 5, the drain electrode 6, and the gate electrode 7 were to beformed was etched by photolithography, thereby obtaining a SiN pattern.

Then, the source electrode 5 and the drain electrode 6 were formed.Specifically, first, a multilayer metal pattern of Ti/Al/Ni/Au (filmthickness thereof is 25/75/15/100 nm) was formed at the predeterminedlocations for formation by the vacuum deposition method and thephotolithography process, thereby forming the electrodes 5 and 6. Afterthat, for improved ohmic property of the source electrode 5 and thedrain electrode 6, heat treatment was performed for 30 seconds at 800°C. in a nitrogen gas atmosphere.

Subsequently, the gate electrode 7 was formed. Specifically, a Schottkymetal pattern of Pd/Au (film thickness thereof is 30/100 nm) was formedat the predetermined locations for formation by the vacuum depositionmethod and the photolithography process, thereby forming the gateelectrode 7.

Finally, the substrate was cut into pieces on a device-by-device basisby dicing, thereby obtaining the HEMT devices 20.

The gate-source capacitance of the obtained HEMT device 20 was measured.Table 1 also shows the measurement results of the gate-sourcecapacitance of each sample.

The results shown in Table 1 confirm that a gate-source capacitance of0.5 pF or less is obtained in the HEMT devices of Nos. 1-2 to 1-6, inwhich the resistivity of the Zn-doped GaN layer 2A is 1×10⁶ Ω·cm ormore, and that a mobility as high as 1000 cm²/V·s or more and agate-source capacitance as small as 0.1 pF or less are obtained in theHEMT devices of Nos. 1-3 to 1-5, in which the range of the Znconcentration of the Zn-doped GaN layer 2A is from 1×10¹⁸/cm³ or more to2×10¹⁹/cm³ or less.

It is also confirmed that the gate-source capacitance does not alwaysdecrease sufficiently in the HEMT devices of Nos. 1-1 and 1-2 in whichthe Zn concentration is smaller than 1×10¹⁸/cm³ while the mobilitydecreases in the HEMT device of No. 1-6 in which the Zn concentration islarger than 2×10¹⁹/cm³.

Example 2

In this example, a group 13 nitride composite substrate 10 including aC-containing GaN layer 2B as a base layer 2 was produced, and further,the group 13 nitride composite substrate 10 was used to produce a HEMTdevice 20. Five types of group 13 nitride composite substrates 10 wereproduced on different conditions for the C concentration of theC-containing GaN layer 2B, and HEMT devices 20 for the respectivesubstrates 10 were produced (Nos. 2-1 to 2-5). The production procedurewas similar to that of Example 1 except for that the C-containing GaNlayer 2B, the channel layer 3, and the barrier layer 4 were formedsequentially in a single MOCVD furnace. Then, the characteristics wereevaluated during and after the production.

In the production of the sample on each condition, a conductive GaNsubstrate similar to that used in Example 1 was placed in the susceptorin the reactor of the MOCVD furnace. After that, the internal gas of thereactor was replaced with a nitrogen gas, and then, an atmosphere inhydrogen/nitrogen mixed flow state was formed. Subsequently, thesusceptor was heated to raise the temperature of the substrate. When thesusceptor temperature reached 1100° C., a TMG gas and an ammonia gaswere introduced into the reactor, thereby forming a GaN layer to have athickness of 10 μm. On that occasion, for different C concentrations,the reactor pressure and the gas ratio of group 15 to group 13 weredifferentiated for each sample. Specifically, the reactor pressure wasset to 100 kPa, 50 kPa, 50 kPa, 10 kPa, and 10 kPa for the samples 2-1to 2-5, respectively. The gas ratio of group 15 to group 13 was set to1000, 2000, 500, 200, and 100 for the samples 2-1 to 2-5, respectively.A hydrogen gas was used as the bubbling gas for a metal organic materialand the carrier gas in the formation of the C-containing GaN layer 2B.

After the formation of the C-containing GaN layer 2B, C concentrationthereof was measured by SIMS.

After that, the procedure from forming the channel layer 3 to finallyobtaining the HEMT device 20 was similar to that of Example 1. Themobility and the gate-source capacitance were measured as in Example 1.

Table 2 shows the C concentration and resistivity of the C-containingGaN layer 2B, and mobility and gate-source capacitance of HEMT for eachsample.

TABLE 2 C concentration Resistivity Mobility Capacitance Sample No.[/cm³] [Ω · cm] [cm²/V · s] [pF] 2-1 5 × 10¹⁶ 3 × 10³ 1800 2.5 2-2 8 ×10¹⁶ 3 × 10⁶ 1600 <0.1 2-3 2 × 10¹⁷ 4 × 10⁶ 1550 <0.1 2-4 3 × 10¹⁸ 6 ×10⁶ 1250 <0.1 2-5 5 × 10¹⁸ 4 × 10⁷ 950 <0.1

The results shown in Table 2 confirm that a resistivity of 1×10⁶ Ω·cm ormore is obtained in the HEMT devices of Nos. 2-2 to 2-5 in which therange of the C concentration of the C-containing GaN layer 2B is from8×10¹⁶/cm³ or more and that a mobility as high as 1000 cm²/V·s or moreand a gate-source capacitance as small as 0.1 pF or less are obtained inthe HEMT devices of Nos. 2-2 to 2-4 in which the range of the Cconcentration of the C-containing GaN layer 2B is from 8×10¹⁶/cm³ ormore to 3×10¹⁸/cm³ or less.

It is also confirmed that the gate-source capacitance does not alwaysdecrease sufficiently in the HEMT device of No. 2-1 in which the Cconcentration is smaller than 8×10¹⁶/cm³ and that the mobility decreasesin the HEMT device of No. 2-5 in which the C concentration is largerthan 3×10¹⁸/cm³.

Example 3

In this example, a group 13 nitride composite substrate 10 including anAlGaN layer 2C having a composition Al_(p)Ga_(1-p)N as the base layer 2was produced, and further, the group 13 nitride composite substrate 10was used to produce the HEMT device 20. Six types of group 13 nitridecomposite substrates 10 were produced on different conditions for the Alcompositional ratio p of the AlGaN layer 2C, and HEMT devices 20 for therespective substrates 10 were produced (Nos. 3-1 to 3-6). The productionprocedure was similar to that of Example 1 except for that the AlGaNlayer 2C, the channel layer 3, and the barrier layer 4 were formedsequentially in a single MOCVD furnace. Then, the characteristics wereevaluated during and after the production.

In the production of the sample on each condition, a conductive GaNsubstrate similar to that used in Example 1 was placed in the susceptorin the reactor of the MOCVD furnace. After that, the internal gas of thereactor was replaced with a nitrogen gas, and then, and the reactorpressure was set to 10 kPa, thereby forming an atmosphere inhydrogen/nitrogen mixed flow state. Subsequently, the susceptor washeated to raise the temperature of the substrate. When the susceptortemperature reached 1100° C., a TMG gas, a TMA gas, and an ammonia gaswere introduced into the reactor, thereby forming an AlGaN layer 2C tohave a thickness of 0.2 μm. On that occasion, for different Alcompositional ratios p, the TMA/(TMA+TMG) ratio was differentiated foreach sample. Specifically, the TMA/(TMA+TMG) ratio was set to 0.08, 0.1,0.2, 0.5, 0.98, and 1 for the samples 3-1 to 3-6, respectively. Ahydrogen gas was used as the bubbling gas for a metal organic materialand the carrier gas in the formation of the AlGaN layer 2C.

After that, the procedure from forming the channel layer 3 to finallyobtaining the HEMT device 20 was similar to that of Example 1. Themobility and the gate-source capacitance were measured as in Example 1.

Table 3 shows the Al composition ratio p and resistivity of the AlGaNlayer 2C (base layer), and mobility and gate-source capacitance of HEMTfor each sample.

TABLE 3 Al composition Resistivity Mobility Capacitance Sample No. ratiop [Ω · cm] [cm²/V · s] [pF] 3-1 0.08 2 × 10⁵ 1800 0.6 3-2 0.1 2 × 10⁶1600 <0.1 3-3 0.2 3 × 10⁶ 1550 <0.1 3-4 0.5 6 × 10⁶ 1400 <0.1 3-5 0.98 2× 10⁷ 1050 <0.1 3-6 1 6 × 10⁷ 850 <0.1

The results shown in Table 3 confirm that a resistivity of 1×10⁶ Ω·cm ormore is obtained in the HEMT devices of Nos. 3-2 to 3-6 in which therange of the Al compositional ratio p of the AlGaN layer 2C is 0.1 ormore and that a mobility as high as 1000 cm²/V·s or more and agate-source capacitance as small as 0.1 pF or less are obtained in theHEMT devices of Nos. 3-2 to 3-5 in which the range of the Alcompositional ratio p of the AlGaN layer 2C is from 0.1 or more to 0.98or less.

It is also confirmed that the gate-source capacitance does not alwaysdecrease sufficiently in the HEMT device of No. 3-1 in which the Cconcentration is smaller than 0.1 while the mobility decreases in thedevice of No. 3-6 in which the C concentration is larger than 0.98. TheHEMT device of No. 3-6 where p=1 is the sample including the base layer2 of AlN.

Comparative Example 1

A HEMT device was produced as in Example 1 (also as in Examples 2 and 3)except for that no base layer 2 was formed and the HEMT structure wasprovided directly on the base material 1.

The gate-source capacitance of the obtained HEMT device was measured,which was 50 pF.

This result reveals that providing the base layer 2 as in Examples 1 to3 is effective for reducing the parasitic capacitance of the HEMTdevice.

Comparative Example 2

A 15 mm×15 mm square semi-insulating GaN substrate having a resistivityof 1×10⁶ Ω·cm or more and a (0001) plane orientation was prepared inplace of the conductive GaN substrate used as the base material 1 inExamples 1 to 3 and Comparative Example 1, and the substrate was used toproduce a HEMT device without forming the base layer 2 as in ComparativeExample 1.

During the production, the mobility and the gate-source capacitance weremeasured as in Examples 1 to 3. As a result, the mobility was 1500cm²/V·s and the gate-source capacitance was less than 0.1 pF.

The results above reveal that even in the case where a conductive GaNsubstrate is used as the base material 1 as in Examples 1 to 3, a HEMTdevice suitable for high-frequency applications, having characteristicscomparable to those of the HEMT device having a HEMT structure provideddirectly on the semi-insulating GaN substrate, is obtained by providingthe base layer 2 between the channel layer 3 b and the base material 1on a predetermined condition.

Comparative Example 3

HEMT devices were produced as in Nos. 2-3 and 2-4 of Example 2 exceptfor that after the formation of a C-containing GaN layer 2B as the baselayer 2, a channel layer 3 was formed to have the same C concentrationas that of the C-containing GaN layer 2B. The total thickness of thebase layer 2 and the channel layer 3 was set to 12 The sample producedon the condition corresponding to No. 2-3 will be referred to as No.2-3a and the sample produced on the condition corresponding to No. 2-4will be referred to as No. 2-4a.

During the production, the mobility and the gate-source capacitance weremeasured as in Examples 1 to 3.

Table 4 shows the C concentration and resistivity of the base layer (andthe channel layer), and the mobility and the gate-source capacitance ofHEMT for each sample.

TABLE 4 Sample C concentration Resistivity Mobility Capacitance No.[/cm³] [Ω · cm] [cm²/V · s] [pF] 2-3a 2 × 10¹⁷ 4 × 10⁶ 900 <0.1 2-4a 3 ×10¹⁸ 6 × 10⁶ 750 <0.1

As a result of the comparison between the results of the HEMT devices ofNos. 2-3a and 2-4a shown in Table 4 and the results of the HEMT devicesof Nos. 2-3 and 2-4 in Example 2 shown in Table 2, the gate-sourcecapacitance was reduced to less than 0.1 pF similarly to the HEMTdevices of Nos. 2-3 and 2-4, but the mobility fell below 1000 cm²/V·s.

These results indicate that to obtain a HEMT device suitable forhigh-frequency applications, it is preferable to form a base layer 2containing a relatively high concentration of C on a conductive basematerial 1 and then provide a channel layer 3 containing a lowconcentration of impurities (substantially containing a relatively lowconcentration of C).

The invention claimed is:
 1. A group 13 nitride composite substrate,comprising: a base substrate of an n-conductivity type formed of GaN; abase layer located directly on said base material, said base layer beingan AlGaN layer formed of Al_(p)Ga_(1-p)N (0.1≤p≤0.98) and having aresistivity of 1×10⁶ Ω·cm or more; a channel layer located on said baselayer, said channel layer being a GaN layer having a total impurityconcentration of 1×10¹⁷/cm³ or less; and a barrier layer located on saidchannel layer, said barrier layer being formed of a group 13 nitridehaving a composition Al_(x)In_(y)Ga_(1-x-y)N (0≤x≤1, 0≤y≤1), wherein thethickness of the barrier layer is 5 to 30 nm.
 2. A method formanufacturing a group 13 nitride composite substrate, comprising: a baselayer forming step of forming, directly on a base substrate of ann-conductivity type formed of GaN, a base layer being an AlGaN layerformed of Al_(p)Ga_(1-p)N (0.1≤p≤0.98) and having a resistivity of 1×10⁶Ω·cm or more; a channel layer forming step of forming, on said baselayer, a channel layer being a GaN layer having a total impurityconcentration of 1×10¹⁷/cm³ or less; and a barrier layer forming step offorming, on said channel layer, a barrier layer formed of a group 13nitride having a composition Al_(x)In_(y)Ga_(1-x-y)N (0≤x≤1, 0≤y≤1).